Syntactic tunnel core

ABSTRACT

A technique and tool are provided for repairing damaged areas of honeycomb structures. The tool includes a plurality of rods axially translatable into the damaged area. Foam material is inserted about the rods and allowed to cure. The rods are later withdrawn leaving a porous core of material. Sleeves may be provided to receive and support the rods as they translate into the damaged area. The sleeves may remain in the damaged area after the rods are withdrawn into the sleeves.

CROSS-REFERENCE TO RELATED APPLICATIONS

(Not Applicable)

STATEMENT RE: FEDERAL SPONSORED RESEARCH/DEVELOPMENT

(Not Applicable)

FIELD OF INVENTION

The present invention relates to an apparatus and technique useful torepair honeycomb core structures, such as used in unmanned aircraft,small submersibles and the like.

BACKGROUND OF THE APPLICATION

Many contemporary aircraft have honeycomb sandwich structures formed inthe wing leading edges. In the course of use the leading edges maysustain damage, e.g. from bird strikes and accidents during maintenance.

In some cases a honeycomb core may be crushed proximate the point ofimpact, but the outer skin may not show any visual damage at thatlocation. The resulting core shear failure may extend over a larger areathan the original damaged area. Such unseen core damage may result inthe outer skin buckling and delaminating under stress.

Repair of the damage may require skin and core repair operations, eitherin the field or in a repair depot. A replacement core specific to thatwing must be matched to correctly restore the aircraft's integrity.However, maintaining a stock of cores specific to each aircraft islogistically challenging, especially during field operations.

In accordance with contemporary repair techniques damaged skin and theadjacent honeycomb core are completely removed to the opposing skin. Areplacement core may be shaped and sanded to fit snugly into the areabeing repaired. Replacement material may be selected to substantiallymatch the original case material. However, conformity of the materialsdoes not insure conformity to the structural and mechanical propertiesof the honeycomb shaped core. One type of replacement material issyntactic foam, an epoxy resin material that contains glassmicro-balloons. This material can be tailored to different densities andproperties. However the material may be too heavy for certainapplications. Moreover, the material is isotropic in nature, i.e. thestrength is equal in all directions, which may be undesirable or atleast differ from the honeycomb properties of the adjacent structure.

For reference, several prior art methods to repair aircraft laminatesare described below. U.S. Pat. No. 6,149,749 discloses a patch used tocover the damaged area, without any replacement of the originalstructure, including the lightweight structure between the two outerskins. The patch has apertures that allow air to be removed from belowthe patch thereby improving the adhesion.

U.S. Pat. No. 4,961,799 discloses a method for repairing damaged areasby bolting or bolt bonding a repair patch on the outer surfaces of thedamaged material. This method does not provide a matching replacementhoneycomb structure.

U.S. Pat. No. 5,741,574 discloses a truss reinforced sandwich structurethat uses fiber bundles or tows cured in very small diameters; thepreferred method of the attachment being stitching the foam core withinthe face sheets or laminates. U.S. Pat. No. 3,328,218 discloses usingrigid filaments to manufacture the core structure. In both referencesthe filaments must be fastened to the outer plies. Moreover, thereferences require the foam to be under compressive pressures of 50 to90 psi.

U.S. Pat. No. 5,612,117 discloses an anchoring means comprising aninsert introduced into a hole in the skins and held in place by the coreto provide an anchoring of the entire structure.

U.S. Pat. No. 5,773,121 describes a syntactic form core structure thatis produced using a powdered resin instead of a liquid resin. In orderto improve or modify characteristics of the resultant structure, choppedfibers or pre-built honeycomb structure is added to the powered resin,before curing.

U.S. Pat. No. 5,547,629 discloses a method using rubber mandrels tofabricate hollow structures such as a wing. After the two skins arecompressed together, with the mandrels in the hollow cavities providingsupport, the mandrels are pulled from one end. As they stretch, theycontract and can be removed from the molded part. No foam honeycombstructure is introduced during the molding process.

U.S. Pat. No. 5,868,886 discloses the use of Z-pins to provide amechanical link between the patch/parent structures. It suggests removalof the pins, leaving holes that are filled when the patch material isintroduced into the repair area. Small pin sizes are used in order toreduce associated structural degradation caused by fiber breakage due toconcentrated stress.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved method forrepairing structures having outer skins and an inner honeycomb corestructure.

It is another object of this invention to provide a repair method thatcan be implemented in-site or at a facility to separate from theaircraft.

It is an additional object of this invention to reduce the weight of thesyntactic foam by displacing a part of the volume of foam with airtunnels.

It is a further object of this invention to regulate the structural ormechanical characteristics of the replacement core by selection of thecore angle(s), number of layers and choice of materials. It is a furtherobject of this invention to reduce the inventory of core specific partsthat must be matched with a particular aircraft, which creates alogistic nightmare, especially during field operations.

In order to reduce the inventory of specific foam cores, a small numberof tools can be kept that will allow syntactic tunnel cores to befabricated as needed. The tooling can be used to produce these syntactictunnel cores both off or on the vehicle. The tooling utilizes metal rodsthat are removable and are held in place with at least 2 rod guideplates. The mechanisms, to hold the tooling in place for repair workboth off and on the vehicle, may be the same depending on the area to bereworked.

If the repair is to be accomplished off aircraft, then the first step istypically to remove the damaged skin and inner honeycomb core. Thisdamaged area is normally removed to the opposite skin. If both skins aredamaged, one skin is typically repaired first. The tooling that holdsthe removable rods may be positioned above a container or tray. Thistray is typically deeper and larger than the scraped out area.

The removable rods are placed into the aligning holes in the rod guideplates and positioned in the cavity created in the tray. The syntacticfoam may be introduced into the cavity, surrounding the removable rods.Once the foam has cured, the removable rods may be removed and thetooling is extracted. The top and bottom surface of the replacementsyntactic tunnel core is now shaped to match the final surface shapesand the original core shape. The shaping of the replacement tunnel coreinclude matching the physical shape of the scraped outer area. The outerskin can now be repaired using contemporary skin replacement techniques.

The core tunnels made from the space taken up by the removable rods mayall be the same geometric shape or vary across the replacement tunnelcore. The most common shapes are circular or hexagonal cylindricaltunnels.

The shape of the tunnels may be a series of graduated sectional profilesnarrowing in the direction of the tunnel depth. Preferably there are noundercuts that may damage the tunnel shapes upon the extraction of theremovable rods.

The angle of the tunnels may typically be perpendicular to the directionof the load of the structure or outer surface or may be offset from thebeing perpendicular. Alternatively, to implement selective structural ormechanical features, the tunnels may be bored at other angle/gradients.A 5-degree offset, for example, will increase the shear characteristics.

During the process, the metal rods may be heated or cooled in accordancewith predetermined temperature probes, to regulate the final structuralcharacteristics or speed up the curing process.

The technique and tool used to implement repaired damaged honeycombstructures is as follows. The tool includes a plurality of rods axiallytranslatable into the damaged area. Foam material is inserted about therods and allowed to cure. The rods are later withdrawn, leaving a porouscore of material. Sleeves may be provided to receive and support therods as they translate into the damaged area. The sleeves may remain inthe damaged area after the rods are withdrawn into the sleeves.

Rods may be formed to have various cross-sectional areas, such ascircular, hexagonal, or other geometric shapes. Rods may be insertedinto the area at different angles relative to the load flow in thedamaged area. The particular angle at which the rods are inserted intothe damaged area may be selected to increase the sheer strength or otherproperties of the honeycomb structure, or to match the properties of theparticular remaining structure.

In one embodiment the rods may be heated, or the temperature otherwiseregulated in order to facilitate curing of the foam material to producedesired properties.

In another embodiment the foam may be inserted in sequentialapplications to define a series of separate layers, wherein each layermay be formed of different material, and/or cured in accordance withdifferent profiles, in order to achieve a desired set of core/structureproperties.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is an illustration of the parts of an aircraft structure.

FIG. 2 contains views of two types of honeycomb structures.

FIG. 3 is an illustration of a removable pin with multiple geometricshapes.

FIG. 4 is a view of a damaged honeycomb section after removal of thedamaged honeycomb section

FIG. 5A is a view of the tooling that is utilized to fabricate thesyntactic tunnel core off of the aircraft.

FIG. 5B is a view of the tooling that is utilized to fabricate thesyntactic tunnel core on the aircraft.

FIG. 6 is an illustration of a removable plug and insert.

FIG. 7 is a view of an off-aircraft repair plug.

FIG. 8 is an illustration of the placement of the plug into the removeddamaged honeycomb area.

FIG. 9 is a top view of the replacement honeycomb plugs in the aircraftstructure.

DETAILED DESCRIPTION OF THE INVENTION

The aircraft or vehicle structure 10 is shown in FIG. 1. The structure10 is made up of two exterior skins 11 and 13. Sandwiched between theskins 11 and 13 is a honeycomb structure 12. Skin 11 is illustrated in atranslucent rendering so that the honeycomb structure 12 can be seen.This honeycomb structure 12 is used to not only reduce the weight of theoverall structure 10, but also to provide the required characteristicsfor a specific application. The tunnels or air pockets of the honeycombstructures can be of any geometric shape.

FIG. 2 shows two different honeycomb structure geometric shapes from twoviewing angles 20. The honeycomb views, 23 and 24 are from differentelevations, based on a circular, cylindrical rod. The honeycomb views,21 and 22, are based on a hexagonal cylindrical rod.

The characteristics of the honeycomb structure can be made directional,whether mechanical, electrical or dielectric, by controlling the amountof holes, their sizes, their geometric shape, their sectional views,their direction, syntactic resin wall thickness and strength,composition of the syntactic material and/or temperature/pressure curingprofiles. The shape of the tunnels, and their angles to the direction ofthe load of the honeycomb structure can further be varied across theface of the repaired area.

FIG. 3 illustrates a removable rod 25, that includes multiple geometricshapes on a single rod. It shows a rod with three geometric shapes, alarger circle 26, a smaller circle 27 just below the larger circle 26,and a hexagonal shape 28 just below the smaller circle 27. Draft may beadded to any of the geometric shapes. The only design preference is thatthe removable rod be capable of being pulled out of the cured honeycombcore. Undercuts could damage the core, by scraping out part of the core.

It is also within the scope of this invention to use layers of syntacticfoam, with each having different properties, to introduce structuralgradients or otherwise improve matching to the surrounding honeycombstructure.

FIG. 4 shows an area of the aircraft structure 30 that was damaged, andthe repair has been started. Though hitting an object, such as by a birdor a projectile, could damage both skins 31 and 32, FIG. 4 assumes thatonly the top skin 31 is damaged. First, the damaged portion of the skin31 is removed as show as a circle of material missing from skin 31. Thehoneycomb internal structure 33 is thereby exposed. The damaged area isnormally removed down to the undamaged skin 32 and the removed area 34is hexagonal in the shape for clarity. It is also possible that thedamaged area of the honeycomb structure 33 will not require the entireremoval of the honeycomb structure down the other skin 32 surface.

Now that the damaged honeycomb area 34 has been removed, a replacementmust be fabricated. This can be accomplished offsite or directly on thevehicle. The decision is based on several factors, including thelocation of the structure being repaired. In a repair depot, when roomsafety is more readily insured, the fabrication of the syntactic tunnelcore will most likely be performed on the vehicle. In the field,especially in war theater, the fabrication of the syntactic tunnel corewould more likely be accomplished off the vehicle.

FIG. 5A shows the fabrication tooling 40 for a syntactic tunnel corethat is fabricated offsite or off the aircraft. The fabrication tool 40basically consists of a housing including at least guide plates 42 and43 and a number of extendable rods 41. The rods 41 in FIG. 5A aretypically of circular, cylindrical shape, and adapted for axial movementwithin the guide plates. As will be apparent to those in the field, therod shapes may alternately be of other geometric shapes as shown in FIG.2 and FIG. 3. The outer mechanism used to position the plates above thework area may be any of a number of different mechanisms that are wellknown in the industry.

The removable rods 41 may be positioned into a container or tray 44 ofan appropriate depth, typically at least as deep as the removed area 34,as shown in FIG. 4. Syntactic foam is then placed into the remainingcavity of the tray 44 and allowed to cure. If required, the removablerods 46 can be heated or cooled to reduce the cure time or to maintain atemperature to achieve selected properties in the cured foam. The rods46, are then removed leaving a replacement syntactic tunnel coretray-shaped structure. This syntactic tunnel core structure may beshaped to fit into the cavity 34 of FIG. 4. The final shape of thesyntactic tunnel core 55 mates with the cavity 65 is shown in FIG. 8.The mated replacement honey tunnel core and original honeycomb core isshown in FIG. 9.

FIG. 5B shows the fabrication tooling 44 for a syntactic tunnel corethat is fabricated on aircraft. The fabrication tooling 45 is basicallyconsisting of at least 2-rod guide plates 47 and 48 and a number ofremovable rods 46. The rods 46 in FIG. 5B are a circular, cylindricalshape. The shape may be other geometric shapes as shown in FIG. 2. Therods or rod guide plates may, depending on the honeycomb design, be madeup of several sections that are mechanically fastened together. Theplate sections may contain aperture patterns that are different from theother plate sections, so that the properties of the final syntactictunnel core replacement is better matched to the original honeycombcore. Once the fabrication tooling 45 is in place, the syntactic foammaterial can then be poured into the remaining cavity of the removedarea 34. The syntactic foam is allowed to cure. If required, theremovable rods 46 can be heated or cooled at selected thermal profilesto reduce the cure time, or to regulate the temperature to achievedescribed properties in the cured foam. The rods 46 are removed, leavinga replacement syntactic tunnel core structure 73 as shown in FIG. 9.

FIG. 6 illustrates a rod set 51. Sleeve 53 is placed on the first end ofthe rod 52, which will have syntactic foam material placed around it.The sleeve 53 may remain in the tunnel core after removal of rod 52, anymay be thermally or otherwise controlled to provide additional desiredcore properties. The sleeve 53 may be made of Teflon or any materialthat will provide desired characteristics.

FIG. 7 illustrates a completed off aircraft fabricated syntactic tunnelcore 56. The shape of the edges 57 are fabricated to fit properly withinthe cavity in the repair area 65 as shown in FIG. 8.

The off aircraft fabricated replacement syntactic tunnel core 56 may beplaced into the cavity 65 of the original honeycomb core 63. As shown inFIG. 8 the shape defined by the edges of core 61 mate with the shapedefined by the edges of the cavity 65. The procedure to fasten the core61 to the undamaged skin 64 may then proceed in accordance with standardrepair procedures.

FIG. 9 shows the top view of a repaired honeycomb area 70. The repairedarea may have been fabricated off aircraft or on aircraft. Within therepaired area 70, the dark hexagonal shaped area is the replacementsyntactic tunnel core 73 of the present invention. The lighter coloredsyntactic tunnel core 72 is the original undamaged tunnel core. Thebottom skin 74 supports the bottom of the replacement tunnel core 73.The upper surface of the replacement core 73 must be shaped to match theupper surface shape of the original core 72. Once the surfaces areuniform between the two cores 72 and 73, then a patch can be added toclose the opening hole 75 in the damaged skin 71. The patch is notshown.

The shear strength of the core with tunnels perpendicular to the skinsis low. If desired, the rods 41 or 45 can be positioned at an angle thatis not perpendicular to the load path direction. Even a 5-degree offsetwill increase the load angle. The angle selected is based on the desiredcharacteristics.

It is understood that although the above represents several embodimentsof the invention, the invention may take a wider variety of embodimentsintended to effect alternate designs or additional features. Suchembodiments are within the scope and spirit of the present invention.

1. A replacement core fabrication tool for fabricating a replacementhoneycomb core corresponding to a damaged area of a vehicle honeycombcore comprising: a. first and second guide plates, the guide plates eachhaving a plurality of apertures extending therethrough; b. a pluralityof independently extendable rods slideably disposable throughcorresponding apertures of the first and second guide plates to aselected depth into the damaged area to conform distal ends of the rodsto a contoured inner surface of the damaged area, and to conform hollowareas within a resin body disposed within the damaged area to ahoneycomb configuration of the honeycomb core; and c. a plurality ofsleeves slideably disposed over distal end portions of the plurality ofindependently extendable rods for disposing the sleeves in the resinbody while the distal end portions of the rods are pulled out of theresin body.
 2. The tool as recited in claim 1 wherein the apertures areof substantially the same geometric shape and size.
 3. The tool asrecited in claim 1 wherein the apertures are substantially hexagonal inshape.
 4. The tool as recited in claim 1 wherein the apertures aresubstantially circular in shape.
 5. The tool as recited in claim 1wherein the first and second guide plates are horizontally translatablewith respect to each other and the angle of the rods with respect to atleast one guide plate is skewable to vary the angle of the rods relativeto the damaged area.
 6. The tool as recited in claim 1 furthercomprising a second plurality of the independently extendable rodsslidably disposable through corresponding apertures of the first andsecond guide plates to abut a surface of the vehicle adjacent thedamaged area.
 7. The tool as recited in claim 1 further comprising guideplate sleeves disposed about at least a portion of the rods, to receiveand support the rods as the rods extend through the guide plates.
 8. Thetool of claim 1 wherein the rods are temperature-controlled.
 9. The toolof claim 1 wherein the distal end portion of each rod is isolated fromdistal end portions of adjacent rods.
 10. The tool of claim 1 whereinthe rods are each independently translatable relative to the plate. 11.The tool of claim 1 wherein each distal end portion of the rods has anarrowing profile in a direction toward the distal end of the rod. 12.The tool of claim 1 wherein each distal end portion of the rods has agraduated sectional profile narrowing in a direction toward the distalend of the rod.
 13. The tool of claim 1 wherein each distal end portionof the rods has a tapered configuration.
 14. The tool of claim 1 whereineach distal end portion of the rods has a narrowing profile in adirection of the depth into the damaged area.
 15. The tool of claim 1further comprising: a heater disposed adjacent to the plurality of rodsand in thermal communication with the plurality of rods; and acontroller in communication with the heater, the controller operative tocontrol the heater to heat the plurality of rods to a selectedtemperature.